I. Field of the Invention
This invention relates to rotatable liner hangers for setting successive sections of casing liner within a well bore and, in particular, to a liner hanger which incorporates double slip cones and double bearings for improved weight distribution of the liner.
II. Description of the Prior Art
Liner hangers have been used for many years to attach an inner string of well pipe or casing liner to the lower end of a larger diameter well pipe or casing liner already set within a well bore. After hanging the liner, the liner will usually be cemented within the well bore by circulating a cement slurry through a setting string and the interior of the liner and upwardly about the exterior of the liner. Typical liner hangers include at least one set of slips with slip elements which wedge on a slip cone to grip the interior surface of the outer casing. The slip elements are arranged to be expanded into gripping engagement with the interior of the larger casing by the underlying slip cone. The slip cone travels beneath the slip elements as a result of mechanical or hydraulic actuation of the tool. In many applications, the casing liners are long and heavy requiring additional slip area to assure effective attachment of the liner. Such attachment can be carried out by large slip assemblies or a plurality of spaced apart slip assemblies.
Because of the nature of the slurry used to cement the casing liners in place, it has been found that the rotation of the liner facilitates proper cementation. As a result, rotatable liner hangers were developed to ensure the flow of cement about the casing liner. Such rotatable liner hangers may utilize a rotatable bearing assembly in conjunction with the slip assembly. The bearing allows rotation of the liner independently of the set hanger during cementation. However, due to the increased length of some liners and a desire for additional fluid by-pass around the liner hanger multiple slip and cone arrangements have been used. It has been found that with some multiple cone rotatable liner hangers, a disproportionate load was being placed on the upper slip assembly and its associated bearing. This resulted in increased wear and failure of the rotatable liner hanger.
Liner hangers have been developed which comprise individual hanger units which can be connected in series to form a multiple bearing or a multiple cone liner hanger. However, because individual units must be manufactured and stocked, such a system is prohibitively expensive. Moreover, since the units operate independent of each other the load may not be evenly distributed between the individual liner hanger units. The separate units are also, at times, difficult to set with varying densities of cement slurry and mud displacement since multiple shear pins must be accurately and sequentially sheared to set the slips.